Boilers are combustion devices which convert energy stored in fuel into usable heat and energy. Although boilers vary in sophistication, a simple boiler comprises a housing which contains a combustor and a heat exchanger. Typically, combustion is supplied by a series of burners which burn a fuel, such as ground, pulverized, or slurried coal, or fuel oil. A flame path is controlled by primary and secondary air and extends into the interior of the boiler. Traversing the boiler interior is at least one heat exchanger. Typically, this takes the form of long fluid-containing tubes traversing the interior of the housing adjacent to or above the flame path. The tubes feed a larger volume steam separator and a steam drum. Heat rising from the flame path superheats the fluid inside the tubes. Energy is recovered in the steam drum as useful heat through a steam outlet. The stack gas, having given up its useful heat, is expelled from the housing to pollution control devices and then to atmosphere.
For the most efficient operation, the flame path should be controllably defined so that secondary air can be appropriately drafted to cause complete combustion of the fuel. One fuel used to generate combustion is a solid or particulate fuel, such as coal, which is generally pulverized to increase its surface area for quick and large volume combustion. Unfortunately, in the case of slurried coal, mechanical processes of particle classification produce particles with a range of sizes and hence different surface area-to-volume ratios. Hence, some particles will reside in the flame path longer than others before combustion occurs to completion. As a result, the flame path is poorly defined. This makes it necessary to draft a relatively large volume of secondary air over a relatively large combustion area to assure complete combustion. However, any secondary air, above up to about 15% over that which is required for stoichiometric combustion, represents dilution of heating and a penalty to efficiency. Moreover, as pollutants must be removed from stack gas before discharge into the atmosphere, excess secondary air increases the volume of air which must be treated and operating costs.
In addition to varying the volume of secondary air, additional firing of the fuel igniters is required to maintain complete combustion. However, this requires energy and penalizes efficiency.
In the case of oil, atomization is required for large-volume rapid combustion. Nevertheless, conventional atomization technology, such as a spray injection system, produces particles with a range of sizes and surface area-to-volume ratios, which increase secondary air requirements above up to about 15% over that required for stoichiometric combustion.
"Dry" coal is typically ground and/or pulverized to prepare it for combustion. Although the particle sizes are sufficiently uniform, varying amounts of surface moisture are also present and must be removed, the heat being provided by the boiler. This represents a loss in efficiency. If the moisture is not removed, the fuel behaves as if the particle size were not uniform.
It would be desirable to provide a fuel of relatively uniform size and moisture content so that the flame path can be optimally controlled, with the volume of secondary air optimized, and refiring of the igniters minimized.
The heat exchanger typically comprises a series of elongate tubes containing the heat exchange fluid, typically water, which is heated by contact with stack gas rising from the flame path. Soot and other imperfectly combusted materials tend to become deposited on the heat exchange tubes. This tends to insulate the tubes and lower the efficiency of heat transfer.
To address this problem, sootblowers are provided to blow compressed air over the tubes to clean them on a periodic basis. However, as the air is taken from an external source, it is relatively cold and dilutes the efficiency of heat exchange. A copious volume of soot and slag is released during sootblowing and is discharged directly to the atmosphere, often bypassing pollution control equipment. Thus, sootblowers reduce operational efficiency and are environmentally objectionable.
It would be desirable to expose the heat exchange surfaces as closely as possible to the flame path without the disadvantages of sootblowers.
The boilers operate most efficiently if the exhaust temperature is in a range of about 300.degree. to about 400.degree. F. However, as fuel is combusted to provide the heat source in the boiler, pollutants, such as sulphur and nitrous oxide (NOX), are commonly present in the stack gas. Their discharge into the atmosphere is environmentally unacceptable and State and Federal regulations now require stack gas to be processed through pollution control devices before leaving the plant. Generally, these take the form of selective catalytic reduction systems or alkali chemical spray systems.
In order for stack gas to be legally disposable into the atmosphere, the pollution control devices must operate to a high degree of chemical efficiency. These require a relatively high stack gas temperature. As a result, boilers must be designed to extract less usable heat from the exhaust gas, sacrificing efficiency. Furthermore, pollution control devices, such as selective catalytic reduction systems, tend to clog with minute particulate matter, which markedly reduces catalyst lifetime far below that determined by a simple rate of catalyst poisoning.
For alkali processes for sulfur removal, chemical reaction is enhanced by an intimate mixing of exhaust gas with the alkali solution. However, the flow of stack gas is, at times, insufficient for complete mixing.
It would be desirable to operate boilers at a low stack gas temperature for greatest extraction of usable heat, yet at the same time, provide stack gas at a sufficiently elevated temperature for proper operation of emission control systems.